Sunspot cycle: A driven nonlinear oscillator?

A property of nonlinear oscillators-mutual dependence between their instant aneous amplitude and frequency-is tested in the yearly and monthly records of the sunspot numbers using the histogram-adjusted isospectral surrogate d ata and the Barnes model as the autoregressive moving average surrogates. T he instantaneous amplitudes and frequencies are obtained by means of the an alytic signal approach using the discrete Hilbert transform. In several tes ts the amplitude-frequency correlation has been found significant on levels ranging from p < 0.03 to p < 0.07, which supports the hypothesis of a driv en nonlinear oscillator as a mechanism underlying the sunspot cycle.